1. Field of the Invention
This invention relates to an apparatus for refractive correction utilized to ablate corneal tissue to reshape the corneal surface, and more particularly, to apparatus for corneal re-profiling by laser ablation for correction of myopia, hyperopia, astigmatic myopia and astigmatic hyperopia conditions.
2. Description of Related Art
With reference to FIG. 1, a schematic illustration of a normal eye is shown, wherein light rays 10 pass through the cornea 12 of eye 14 through lens 16 to the retina 18. In a normal eye, the focal point 20 of the light rays occurs on the retina 18 for normal vision.
Common vision defects occur when the focal point 20 is not disposed on the retina 18. For example, as shown in FIG. 2, myopia occurs when the focal point 20 is disposed in front of the retina 18. Myopia may be corrected as shown in FIG. 3 by using conventional methods by ablating tissue 24 of the cornea 12 so as to increase the radius of curvature of cornea 12 to shift the focal point 20 to be on the retina 18.
With reference to FIG. 4, hyperopia occurs when the shape of the cornea does not permit the light rays 10 to focus on the retina 18. Instead, the focal point 20 of the light rays 10 in an eye suffering from hyperopia is disposed behind the retina 18. As shown in FIG. 5, hyperopia can be corrected by ablating tissue 24 of the cornea 12 so as to decrease the radius of curvature of the center of the cornea 12, shifting the focal point 20 to be on the retina 18.
FIGS. 6 and 6A illustrate myopic astigmatism. As shown in FIG. 6, x-direction light rays 10' and y-direction light rays 10" pass through the cornea 12 and lens 16 to the retina 18. However, due to the shape of the cornea 12, the x-direction light rays 10' focus at focal point 20', while the y-direction light rays 10" focus at focal point 20" in front of the retina 18, causing vision to be blurred. FIG. 6A is a three-dimensional illustration of the light rays 10' and 10" passing through the cornea 12 and showing the location of the focal points 20' and 20". Myopic astigmatism can be corrected by re-profiling the surface of the cornea 12 to obtain a single focal point on the retina 18. It can be appreciated that in hyperopic astigmatism, the focal points 20' and 20" of the x-direction and y-direction light rays, respectively, are behind the retina. Hyperopic astigmatism may also be corrected by corneal re-profiling.
When correcting conditions such as, for example, myopia, hyperopia and/or astigmatism, the eye may be ablated in thin layers by an excimer laser or the like to achieve the desired correction. For simple myopic correction, the shape of each layer of the ablation is represented by the equation for a circle. Parabolic, spherical or other mathematical models could be used. For a small laser beam to ablate each layer of the corneal tissue, the laser beam must be scanned across the surface of the eye, inside of and on the curve defining the zone of ablation. Many approaches can be used to scan each layer. The scanning process may be defined as linear, circular, random multi-beam, or other useable method.
However, when employing a mathematically modeled curve to define each layer of ablation in performing laser vision corrections, and when the center of each laser pulse is positioned inside of or at the boundary of the curve, the effective ablation area for each layer may be smaller than that defined by the actual curve modeled because of the incremental movement of the laser. Thus, the correction may not be as effective as theorized.
Accordingly, there is a need to provide an apparatus that controls laser pulse center point positions effectively throughout a layer of ablation.